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Bonding. Forces that hold molecules together. Learning Goals. Students will: Understand how ionic and covalent bonds form (review) Understand electronegativity and polar covalent bonding Lewis dot diagrams and Lewis structures
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Bonding Forces that hold molecules together
Learning Goals Students will: • Understand how ionic and covalent bonds form (review) • Understand electronegativity and polar covalent bonding • Lewis dot diagrams and Lewis structures • Understand how polar molecules form and how this affects the properties of molecules
Success Criteria • Students will: • Use electronegativity to understand polar covalent bonds and polar bonds • Use Lewis Dot Diagrams to explain bonding • Use polarity to explain the physical properties of ionic and covalent compounds
REMEMBER Grade 10 Chemistry when we made the models of molecules • The atoms were stuck together with plastic connectors. These plastic connectors represent chemical bonds. • These connectors (chemical bonds) represent the forces that hold molecules together. Compounds are formed when atoms bond together. Plastic connectors used to represent chemical bonds. Methanol molecule Methane molecule Carbon dioxide
Types of Bonds • Bonds form because molecules are attracted to each other. There are three types of bonds: • Ionic Bonds– form when electrons are transferred from one atom to another. • Covalent Bonds – form when two atoms share electrons. • NEW IDEA! - Polar Covalent Bonds – this a combination of the two types of bonds found above. Two atoms share electrons but the electrons are not shared equally (one atom pulls the electrons closer to itself)
Ionic Bond • In this diagram we can see that the sodium atom transfers one of its electrons to the chlorine atom. • This makes the sodium atom positively charged and the chlorine atom negatively charged. • Since atoms with opposite charges attract each other, an electrostatic force holds them together in an ionic bond.
Ionic Bond • In this diagram we can see that the sodium atom transfers one of its electrons to the chlorine atom. • This makes the sodium atom positively charged and the chlorine atom negatively charged. The two atoms attract eachother to form an ionic bond.
Ionic Bond • In this diagram we can see that the calcium atom transfers two electrons - one each to the chlorine atoms. • This makes the calcium atom positively charged and the chlorine atoms negatively charged.
Covalent Bond • In this diagram we can see an oxygen atom sharing electrons with two hydrogen atoms to form H2O or water. • The atoms are held together because they share a pair of electrons. The shared pair of electrons is called a covalent bond. Covalent bonds
Covalent Bond • In this diagram we can see a carbon atom sharing electrons with two oxygen atoms to form CO2. • Two shared pairs are required for each bond – a double bond. • A comparison between Bohr Diagrams and Lewis Dot Diagrams is shown.
Covalent Bond • In this diagram we can see a carbon atom sharing electrons with two oxygen atoms to form CO2. • Two shared pairs are required for each bond – a double bond. • A comparison between Bohr Diagrams and Lewis Dot Diagrams is shown.
Multiple bonds • Atoms can transfer or share one, two or three electrons. These multiple bonds are usually stronger. Ethyne (C2H2) Triple bond between carbons Ethane (C2H6) Single bond between carbons Ethene (C2H4) Double bond between carbons
How can we tell which type of bond is being used? 1) Look at the chemical formula Ionic Compounds – metal + non-metal Covalent Compounds – non-metal + non-metal 2) Look at the Physical properties Ionic Compounds – crystalline, brittle, high melting points, often soluble (they dissolve) Covalent Compounds - gases, liquids or soft solids (plastics), low melting points, not often soluble 3) Use the Electronegativity Chart
Determining the Bond Type with Electronegativity • Look at the chart of electronegativity of the atoms. We calculate the difference in electronegativity between atoms that are bonded together. We subtract the electronegativity of one atom from the other. (see next page)
Determining the Bond Type with Electronegativity If the difference is 0.0 to 0.4 – covalent bond If the difference is 0.4 to 1.7 – polar covalent bond If the difference is greater than 1.7 – ionic bond
Polar Molecules • Compounds with ionic bonds and polar covalent bonds usually form polar molecules. • This happens because one atom pulls the electrons closer to itself and becomes negatively charged. The other atom has fewer electrons and becomes positively charged. This means one side of the molecule is a negative pole and the other side is a positive pole. • Compounds with covalent bonds form non-polar molecules since neither atom gains a positive or negative charge.
Water (H2O) is the classic polar molecule. Oxygen has a higher electronegativity (3.5) than hydrogen (2.1). The electronegativity difference is 1.4 and a polar covalent bond forms. This means that the oxygen atom gains a negative charge and the hydrogen gains a positive charge. Water is a polar molecule. Water (H2O)
Polar Molecules • Determine the electronegativity of both atoms. • Note the symbols δ- and δ+, they indicate the slight negative and positive charges associated with each atom in a polar covalent bond only! • This diagram shows the larger electron cloud surrounding the oxygen atom. • Water is polar due to the strong pull on electrons by the oxygen.
Non-Polar Molecules • Compounds with covalent bonds usually form non-polar molecules since neither atom gains a positive or negative charge. • Non-polar molecules mix into each other very easily (not true dissolution) • Grease dissolves into gasoline quite nicely although we don’t recommend gasoline as a cleanser for your clothes. • Soaps are special in that they have a polar and a non-polar side (hydrophilic and hydrophobic) and can dissolve grease and wash them away in water.
Non-Polar Molecules • Some non-polar molecules contain polar covalent bonds, however the symmetrical arrangement of the bonds balances the charge
Polar vs. Non-Polar • Look at both the polarity of bonds and the symmetry of the molecules.
Polar vs. Non-Polar • Look at both the polarity of bonds and the symmetry of the molecules.
Solubility • Polar molecules only dissolve into polar liquids. For example, salt dissolves into water. • Salt (NaCl) is ionic and water is a polar molecule (polar covalent bonds)
Solubility • Non-polar molecules only dissolve into non-polar liquids • Grease dissolves in gasoline • Oil paint dissolves in paint thinner
Solubility • A solution is a uniform mixture of two substances. It is composed of a solvent (the substance that does the dissolving) and a solute (the substance that gets dissolved). • For example Kool Aid is a solution with the drink crystals being the solute and water being the solvent.
Solubility • Water dissolves Sodium Chloride, the polar water molecules surround the NaCl crystals and rip it apart! This causes the Ionic Crystal (NaCl) to Dissociate or Separate in Na+ ion and a Cl- ion.
Miscibility • When a solute and a solventdo not mix, the solute is said to be insoluble. The solute and solvent are immiscible. • When the solvent and solute do mix they are soluble and miscible.
Miscibility • Why is it important to know about polar and non-polar molecules? • Polar molecules dissolve into other polar molecules • Non-polar molecules dissolve into other non-polar molecules but often not as easily! • BUT polar molecules do not dissolve non-polar molecules – they form mixtures. • LIKE DISSOLVES LIKE!
Miscibility • Oil and water do not mix. They are immiscible. (Oil is insoluble in water). Oil is non-polar and water is polar. • Salt dissolves in water. They are miscible. (Salt is soluble in water). • Both salt (NaCl) and water are polar molecules.
Colloids • Colloids are mixtures that look a lot like solutions. They are very fine particles that are non-polar and mix into other non-polar compounds and sometimes water. Whipped Cream Milk Mayonnaise Solid stick deodourant Jello
